1. Field of the Invention
The present invention relates to crystallizing an amorphous silicon film, and, more particularly, to a sequential lateral solidification (SLS) crystallization method.
2. Discussion of Related Art
Polycrystalline silicon (p-Si) and amorphous silicon (a-Si) are often used as active layer materials for thin film transistors (TFTs) in liquid crystal display (LCD) devices. Since amorphous silicon (a-Si) can be deposited at a low temperature to form a thin film on a glass substrate, amorphous silicon (a-Si) is commonly used in liquid crystal displays (LCDs). Unfortunately, amorphous silicon (a-Si) TFTs have relatively slow display response times that limit their suitability for large area LCDs.
In contrast, polycrystalline silicon TFTs provide much faster display response times. Thus, polycrystalline silicon (p-Si) is well suited for use in large LCD devices, such as laptop computers and wall-mounted television sets. Such applications often require TFTs having a field effect mobility greater than 30 cm2/Vs and a low leakage current.
A polycrystalline silicon film is comprised of crystal grains having grain boundaries. The larger the grains and the more regular the grains boundaries, the better the field effect mobility. Thus, a crystallization method that produces large grains, ideally a single crystal, would be useful.
One method of crystallizing amorphous silicon into polycrystalline silicon is sequential lateral solidification (SLS). SLS crystallization uses the fact that silicon grains tend to grow laterally from the interface between liquid and solid silicon. With SLS, amorphous silicon is crystallized using a laser beam having a magnitude that melts amorphous silicon such that the melted silicon forms laterally grown silicon grains upon re-crystallization.
FIG. 1A is a schematic configuration of a conventional sequential lateral solidification (SLS) apparatus, while FIG. 1B shows a plan view of a conventional mask 38 that is used in the apparatus of FIG. 1A. In FIG. 1A, the SLS apparatus 32 includes a laser source 36, a mask 38, a condenser lens 40, and an objective lens 42. The laser source 36 emits a laser beam 34. The intensity of the laser beam 34 is adjusted by an attenuator (not shown) that is located in the path of the laser beam 34. The laser beam 34 is condensed by the condenser lens 40 and is then directed onto the mask 38.
The mask 38 includes a plurality of slits “A” that pass the laser beam 34 and light absorptive areas “B” that absorb the laser beam 34. The width of each slit “A” effectively defines the grain size of the crystallized silicon produced by a first laser irradiation. Furthermore, the distance between the slits “A” defines the size of the lateral grain growth of amorphous silicon crystallized by the SLS method. The objective lens 42 is arranged below the mask and reduces the shape of the laser beam 34 that passed through the mask 38.
Still referring to FIG. 1A, an X-Y stage 46 is arranged adjacent the objective lens 42. The X-Y stage 46, which is movable in two orthogonal axial directions, includes an x-axial direction drive unit for driving the x-axis stage and a y-axial direction drive unit for driving the y-axis stage. A substrate 44 is placed on the X-Y stage 46 so as to receive light from the objective lens 42. Although not shown in FIG. 1A, it should be understood that an amorphous silicon film is on the substrate 44, thereby defining a sample substrate.
To use the conventional SLS apparatus, the laser source 36 and the mask 38 are typically fixed in a predetermined position while the X-Y stage 46 moves the amorphous silicon film on the sample substrate 44 in the x-axial and/or y-axial direction. Alternatively, the X-Y stage 46 may be fixed while the mask 38 moves to crystallize the amorphous silicon film on the sample substrate 44.
When performing SLS crystallization, a buffer layer is typically formed between the substrate and the amorphous silicon film. Then, the amorphous silicon film is deposited on the buffer layer. Thereafter, the amorphous silicon is crystallized as described above. The amorphous silicon film is usually deposited on the buffer layer using chemical vapor deposition (CVD). Unfortunately, that method produces amorphous silicon with a lot of hydrogen. To reduce the hydrogen content the amorphous silicon film is typically thermal-treated, which causes de-hydrogenation, which results in a smoother crystalline silicon film. If de-hydrogenation is not performed, the surface of the crystalline silicon film is rough and the electrical characteristics of the crystalline silicon film are degraded.
FIG. 2 is a plan view showing a substrate 44 having a partially-crystallized amorphous silicon film 52. When performing SLS crystallization, it is difficult to crystallize all of the amorphous silicon film 52 at once because the laser beam 34 has a limited beam width, and because the mask 38 also has a limited size. Therefore, the substrate 38 is typically moved numerous times such that crystallization is repeated at various locations such that the substrate is completely crystallized. In FIG. 2, an area “C” that corresponds to one mask position is called a block. Crystallization of the amorphous silicon within the block “C” is achieved by irradiating the laser beam several times.
SLS crystallization of the amorphous silicon film 52 will be explained as follows. FIGS. 3A to 3C are plan views showing one block of an amorphous silicon film 52 being crystallized using a conventional SLS method. In the illustrated crystallization, it should be understood that the mask 38 (see FIGS. 1A and 1B) has three slits.
The length of the lateral growth of a grain is determined by the energy density of the laser beam, by the temperature of the substrate, and by the thickness of amorphous silicon film (as well as other factors). The maximum lateral grain growth should be understood as being dependent on optimized conditions. In the SLS method shown in FIGS. 3A to 3C, the width of a slit is twice as large as the maximum lateral grain growth.
FIG. 3A shows the initial step of crystallizing the amorphous silicon film 52 using a first laser beam irradiation. As described with reference to FIG. 1A, the laser beam 34 passes through the mask 38 and irradiates one block of an amorphous silicon film 52 on the sample substrate 44. The laser beam 34 is divided into three line beams by the three slits “A.” The three line beams irradiate and melt regions “D”, “E” and “F” of the amorphous silicon film 52, reference FIG. 3A. The energy density of the line beams should be sufficient to induce complete melting of the amorphous silicon film 52. That is, the portion of the amorphous silicon film that is irradiated by the laser beam 34 is completely melted through to the buffer layer.
Still referring to FIG. 3A, after complete melting the liquid phase silicon begins to crystallize at the interfaces 56a and 56b of the solid phase amorphous silicon and the liquid phase silicon. Crystallization occurs such that grains grow laterally. Thus, as shown, lateral grain growth of grains 58a and 58b proceeds from the un-melted regions to the fully melted regions. Lateral growth stops when: (1) grains grown from interfaces collide near the middle section 50a of the melted silicon region; or (2) polycrystalline silicon particles are formed in the middle section 50a as the melted silicon region solidifies sufficiently to generate solidification nuclei.
Since the width of the slits “A” (see FIG. 1B) is twice as large as the maximum lateral growth of the grains 58a and 58b, the width of the melted silicon region “D,” “E,” and “F” is also twice as large as the maximum lateral growth length of the grains. Therefore, the lateral grain growth stops when the polycrystalline silicon particles are formed in the middle section 50a. Such polycrystalline silicon particles act as solidification nuclei in a subsequent crystallization step.
As discussed above, the grain boundaries in directionally solidified silicon tend to form perpendicular to the interfaces 56a and 56b between the solid phase amorphous silicon and the liquid phase silicon. Thus, as a result of the first laser beam irradiation, crystallized regions “D,” “E,” and “F” are formed. Additionally solidification nuclei regions 50a are also formed.
As previously mentioned, the length of lateral grain growth attained by a single laser irradiation depends on the laser energy density, the temperature of substrate, and the thickness of the amorphous silicon film. Typically, lateral grain growth ranges from 1 to 1.5 micrometers (μm).
FIG. 3B illustrates crystallizing the amorphous silicon film 52 of FIG. 3A using a second laser beam irradiation. After the first laser beam irradiation, the X-Y stage or the mask 38 moves in a direction along the lateral grain growth of the grains 58a or 58b (in FIG. 3A), i.e., in the X direction, by a distance that is no more than the maximum length of the lateral grain growth. Then, a second laser beam irradiation is conducted. The regions irradiated by the second laser beam are melted and crystallized as described above. The silicon grains 58a and 58b and/or the nuclei regions 50a produced by the first laser beam irradiation serve as seeds for the second crystallization. Thus the lateral grain growth proceeds in the second melted regions. Silicon grains 58c formed by the second laser beam irradiation continue to gross adjacent to the silicon grains 58a formed by the first laser beam irradiation, and silicon grains 58d grown from an interface 56c are also formed. The lateral growth of these grains 58c and 58d stops when the nuclei regions 50b are formed in a middle section of the silicon region melted by the second laser beam irradiation.
Accordingly, by repeating the foregoing steps of melting and crystallizing, one block of the amorphous silicon film is crystallized to form grains 58e as shown in FIG. 3C.
The above-mentioned crystallization processes conducted within one block are repeated block by block across the amorphous silicon film. Therefore, the large size amorphous silicon film is converted into a crystalline silicon film. While generally successful, the conventional SLS method described above has disadvantages.
Although the conventional SLS method produces large size grains, the X-Y stage or the mask must repeatedly move a distance of several micrometers to induce lateral grain growth. Therefore, the time required to move the X-Y stage or the mask 38 occupies a major part in the total process time. This significantly decreases manufacturing efficiency.
FIG. 4 is a plan view of a mask 60 that is used in another SLS method. The mask 60 has light slits “G” and light absorptive areas “H.” Although the mask 60 is similar to the mask 38 shown in FIG. 1B, the width of the lateral stripe-shaped slits “G” is less than twice the maximum lateral grain growth length. Due to the smaller width of the slits “G” the lateral grain growth stops when the grains generated at the interface between the un-melted regions and the fully melted regions. In contrast to the crystallization described in FIGS. 3A to 3C, solidification nuclei regions 50a and 50b are not formed when using the mask.
The SLS using the mask 60 will now be discussed. As described with reference to FIG. 1A, the laser beam 34 passes through the mask 60 and irradiates the amorphous silicon film on the sample substrate 44. The laser beam 34 is divided into three line beams because there are three slits “G”. Those line beams are reduced by the objective lens 42 to create beam patterns on the amorphous silicon film 52. As crystallization proceeds, the beam patterns move in an X-axis direction. Because of the X-axis directional movement, crystallization is conducted along a length of the beam pattern. As previously described, the X-Y stage 46 or the mask 60 moves by a distance of several millimeters (mm). The larger movement reduces processing time when compared to the SLS method described with reference to FIGS. 3A to 3C.
FIGS. 5A to 5C are plan views showing an amorphous silicon film in the crystallization being crystallized using the mask shown in FIG. 4. It is assumed that the mask 60 has three slits. As mentioned above, the length of lateral grain growth is determined by the energy density of the laser beam 34, the temperature of substrate, the thickness of amorphous silicon film, etc. Thus lateral grain growth of the grains is the maximized under optimized conditions. In FIGS. 5A to 5C, it should be understood that the width of the slits “G” (in FIG. 4) is smaller than twice the maximum length of lateral grain growth.
FIG. 5A shows an initial step of crystallizing the amorphous silicon film. Referring to FIGS. 1A and 5A, the laser beam 34 emitted from the laser source 36 passes through the mask 60 (which replaces the mask 38) and irradiates a first block E1 of an amorphous silicon film 62 deposited on the sample substrate 44. The laser beam 34 is divided into three line beams by the slits “G” The three line beams irradiate and melt regions “I,” “J,” and “K” of the amorphous silicon film 62. Since each of the melted regions “I,” “J,” and “K” corresponds to a slit “G,” the width of the melted regions “I,” “J,” and “K” is less than twice the maximum lateral grain growth. The energy density of the line beams should be sufficient to induce complete melting of the amorphous silicon film.
The liquid phase silicon begins crystallize at the interfaces 66a and 66b of the solid phase amorphous silicon and the liquid phase silicon. Namely, lateral grain growth of the grains 68a and 68b proceeds from un-melted regions to the fully melted regions. Then, lateral growth stops where the grains 68a and 68b collide along a middle line 60a of the melted silicon region. The grain boundaries tend to form perpendicular to the interfaces 66a and 66b. As a result of the first laser beam irradiation, the first block E1 is partially crystallized. Thereafter, by moving the X-Y stage the beam patterns move in the X-axis direction. A second irradiation is conducted and the second block E2 is partially crystallized. The crystallization in the X-axis direction is then repeated to form a third block E3.
As a result of the first to third laser beam irradiations described in FIG. 5A, crystallized regions “I,” “J,” and “K” are formed, each having first to third blocks E1, E2 and E3.
In FIG. 5B, after the first set of laser beam irradiations the X-Y stage or the mask moves in a direction opposite to the lateral growth of the grains 68a or 68b by a distance equal to or less than the maximum length of the lateral growth. Crystallization is then conducted block by block in the X-axis direction. Therefore, the regions irradiated by the laser beam are melted and then crystallized in the manner described in FIG. 5A. At this time, the silicon grains 68a or/and 68b grown by the first to third laser beam irradiations serve as seeds for this crystallization. Silicon grains 68c formed by sequential lateral solidification (SLS) continue to grow adjacent to the silicon grains 68a of FIG. 5A, and silicon grains 68d solidified from an interface 66c are also formed. These grains 68c and 68d collide with each other at a middle line 60b of the silicon regions melted by the laser beam irradiation, thereby stopping the lateral grain growth.
Accordingly, by repeating the foregoing steps of melting and crystallizing the amorphous silicon, the blocks E1, E2 and E3 of the amorphous silicon film become crystallized to form grains 68e as shown in FIG. 5C. FIG. 5C is a plan view showing a crystalline silicon film that resulted from lateral growth of grains to predetermined sizes.
The conventional SLS methods described in FIGS. 3A to 3C and 5A and 5C have some disadvantages. The conventional SLS method takes a relatively long time to crystallize the amorphous silicon film, thereby causing a decrease in manufacturing efficiency. Furthermore, due to the width of the slits of the mask, the length of lateral grain growth is limited.
More rapid crystallization can be achieved using masks having different slit patterns and laser beam scanning in a horizontal direction as shown in FIG. 6. As shown in FIG. 6, a mask 70 includes a plurality of slit patterns 72 that are divided into a first group “M” and a second group “N.” First slit patterns 72a are in the first group “M” and second slit patterns 72b are in the second group “N”. Intervals “O” are between the first slit patterns 72a and between the second slit patterns 72b. Thus, as shown in FIG. 6, each first slit pattern 72a is opposite an interval “O” between the second slit patterns 72b, and each second slit pattern 72b is opposite an interval “O” between the first slit patterns 72a. Referring to FIG. 6, it can be seen that the width of the slit patterns 72 is greater than the interval “O.” The width of the slit patterns 72 should be the same as or less than the maximum lateral grain growth.
Therefore, when the mask 70 or a X-Y stage moves in a transverse direction (i.e., x-axial direction and to the right) after a first amorphous silicon crystallization step, the first slit patterns of the first group “M” are positioned over locations previously covered by the intervals “O.” Accordingly, grains having a desired grain size can be obtained by repeatedly moving the mask 70 in the transverse direction during the amorphous silicon crystallization. Crystallization of amorphous silicon film using the mask 70 will be explained in detail with reference to FIGS. 7A to 7F.
FIG. 7A shows an initial step of crystallizing an amorphous silicon film using the mask of FIG. 6. As described with reference to FIG. 1A, the laser beam 34 passes through the mask 70 (which replaces the mask 38) and irradiates the amorphous silicon film 80 on the sample substrate 44. When applying the laser beam 34 to the amorphous silicon film 80, the laser beam 34 scans along the x-axial direction. Laser beam patterns having the same shape as the slit patterns 72 of the mask 70 partially melt the amorphous silicon film 80 and make first and second melted regions 86a and 86b, respectively, in first and second melted groups “P1” and “P2.” The first and second melted groups correspond to the first and second slit groups “M” and “N”. The energy density of the line beams should be sufficient to induce complete melting of the amorphous silicon film 80 through to an underlying buffer layer.
Still referring to FIG. 7A, after complete melting, the liquid phase silicon begins to crystallize at the interfaces 84a and 84b between the solid phase amorphous silicon and the liquid phase silicon. Namely, lateral grain growth of grains 82a and 82b proceeds from the un-melted regions to the fully melted regions. Then, lateral growth stops in accordance with the width of the melted silicon regions 86a and 86b where the grains 82a and 82b collide along the middle lines 84c of the melted silicon regions. The grain boundaries in directionally solidified silicon tend to form perpendicular to the interfaces 84a and 84b between the solid phase amorphous silicon and the liquid phase silicon. As a result of the first laser beam scanning, the first and second melted groups “P1” and “P2” are partially crystallized. Here, all of the crystallized regions 86 have the same size and shape, and thus, the first partially crystallized group “P1” is the same as, but offset from, the second partially crystallized group “P2.”
Referring now to FIG. 7B, thereafter, by moving the X-Y stage where the substrate is mounted, the beam patterns move in the X-axis direction by the length “Q” of the crystallized regions 86. Thus, the first slit patterns 72a of the first slit group “M” are located over the second partially crystallized group “P2,” and the second slit patterns 72b of the second slit group “N” are located over a new regions of the amorphous silicon film 80. Especially, the first slit patterns 72a is positioned between the second crystallized regions 86b. Thereafter, second laser beam scanning is conducted, and thus, the silicon regions irradiated by the second laser beam are melted and crystallized.
Now referring to FIG. 7C, an overlapped region “R1” which is exposed to the first and second laser beam scanning is completely crystallized to have a predetermined width “T.” Simultaneously, another partially crystallized group “R2” is formed next to the region “R1”. In other words, after the second laser beam scanning and crystallization, new grains having a laterally growing grain length “S” are then formed. Since the new grains 88 continue to grow adjacent to the first grains 82a, the grain length “S” of the new grains 88 is the same as a length from the first middle line 84c (which is formed by the first crystallization) to a second middle line 84d (which is formed by the second crystallization).
After the second laser beam scanning and crystallization, the mask 70 moves again in an x-axial direction for a third laser beam scanning by the length “Q” of the crystallized regions. Thus, the first slit group “M” having the first slit patterns 72a is located over the partially crystallized group “R2,” as shown in FIG. 7D. By a third laser beam scanning and crystallization, the partially crystallized group “R2” becomes a completely crystallized region “R3” as shown in FIG. 7E.
By repeatedly carrying out the foregoing steps of melting and crystallizing, the amorphous silicon film 82 is converted into a polycrystalline silicon film 92 having grains 90 of length “S,” reference FIG. 7F.
However, the conventional SLS method described with reference to FIGS. 1 to 7F has some problems. For example, the SLS method described with reference to FIGS. 3A to 3C (i.e., often referred to as Scan & Step SLS method) takes a rather long time to crystallize the amorphous silicon film, thereby decreasing manufacturing yields and throughput. The SLS method described with reference to FIGS. 5A to 5C (i.e., often referred to as Continuous SLS method) and the SLS method described with reference to FIGS. 7A to 7F (i.e., often referred to as Single Scan SLS method) take a shorter time than the Scan & Step SLS method, but they have limited laser beam patterns widths. Namely, since the width of the laser beam patterns is less than or equal to the maximum length of the lateral grain growth, the grain size is limited. The sizes of the grains formed by the aforementioned methods are shown in Table 1. Table 1 also shows the number of substrates that are processed in accordance with the lateral grain growth length (micrometer; μm) in each crystallization method.
TABLE 1Crystal-lizationLateral Grain Growth (Mm)Method1.752.547101316192230.25Scan &2.12.12.01.91.91.81.71.71.61.5StepContin-52.236.622.913.19.27.05.74.84.23.0uousSingle62.447.432.032.013.910.98.97.56.54.8ScanTo get the results;Exposure area = 1.5 × 25 mm2Laser frequency = 230 HzGlass substrate size = 370 × 470 mm2Stage stepping time = 0.4 sec.Load & Unload time = 10 sec. (per substrate)Maximum length of lateral grain growth = 1 micrometer (μm)Substrate moving distance = 0.75 micrometers (μm)
From the results of Table 1, as the lateral grain growth length becomes larger, the manufacturing yields is reduced. Namely, the larger the lateral grain growth length, the less the throughput.